The invention described herein relates generally to OFDM wireless communications, and more particularly to an OFDM transmitter that pre-compensates digital frequency-domain samples for transmission filter deviations from a flat amplitude and/or group delay across the allocated spectrum.
Orthogonal Frequency Division Multiplexing (OFDM) is a digital multi-carrier modulation technique that uses a plurality of closely-spaced orthogonal subcarrier frequencies to carry data. OFDM operates by dividing a spectrum of transmission data into multiple narrowband sub-channels with a specific spacing termed “orthogonal spacing,” where a fraction of the total data rate specified for the transmission data is modulated onto each sub-channel with a conventional modulation scheme (e.g., quadrature amplitude modulation). With OFDM, different subcarriers are allocated to different mobile devices, which allows several mobile devices to share the available bandwidth. OFDM is deployed or planned for a variety of wireless systems, including IEEE 802.16 (WiMAX), some IEEE 802.11a/g wireless LANs (Wi-Fi), IEEE 802.20 Mobile Broadband Wireless Access (MBWA), and the like.
A practical implementation of an OFDM transmitter presents data symbol values associated with different subcarrier frequencies of a digital input data block, referred to herein as digital frequency-domain samples, to different frequency-domain inputs of an Inverse Discrete Fourier Transform (IDFT) processor to generate a time-domain representation of the desired transmission waveform, referred to herein as a modulated multi-frequency signal stream. The modulated multi-frequency signal stream is converted to an analog stream of modulated symbols. After upconversion and amplification, the resulting analog transmission signal is wirelessly transmitted to a remote device.
OFDM transmitters may be used in networks that employ frequency-scheduling algorithms to accommodate other mobile devices in a spectrum underutilized by a first mobile device. When sharing a spectrum between multiple mobile devices, it is important to minimize the leakage of transmitter energy into unallocated parts of the spectrum, so that a first mobile device's signal does not interfere with a second mobile device's signal, even when the first mobile device is much closer to a network station than the second mobile device.
Known techniques for minimizing energy leakage into unallocated parts of the available spectrum include filtering the signals using digital and/or analog filters. Such filters, however, may also introduce group delay and/or amplitude errors that lead to channel estimation errors at the receiver. For example, energy leakage into unallocated parts of the spectrum is a function of the tails of the transmitted spectrum and the transmitter noise floor. One source of spectral tails and noise floor is the quantizing noise and non-linearities of the digital-to-analog conversion process. Using analog filters that are as sharp as practically possible for the allocated spectrum after digital-to-analog conversion may minimize the noise. Unfortunately, sharp cut-off analog filters tend to exhibit group-delay distortion, whereby the phase versus frequency curve is non-linear. The group delay distortion causes errors in the channel estimation process implemented at the receiver. More particularly, OFDM receivers use pilot symbols having a known phase on selected OFDM subcarriers to estimate the channel. If a transmission filter applies group-delay distortion to the transmitted pilot symbols, the receiver channel estimation process produces a different result than would have been obtained with non-distorted pilot symbols. One solution to the group-delay distortion problem is to use an analog filter having a flat group delay across the allocated spectrum. However, analog filters designed for a flat group delay tend to have a non-flat amplitude over the allocated OFDM spectrum, which causes similar channel estimation problems at the receiver.
Receiver channel estimate errors may cause errors in transmitter operations as well as receiver operations. These errors occur when a transmitter uses channel estimates determined by a collocated receiver to estimate a future transmission channel, e.g., in multiple-input, multiple-output (MIMO) systems. For example, when the uplink and downlink signals use different frequencies, e.g., with Frequency Division Duplex (FDD) systems, other methods may be used to provide the transmitter with knowledge of the transmission propagation channel before transmission. U.S. Pat. No. 6,996,375 to applicant proposes one solution that loops back the received signal to the transmitter to enable the transmitter to figure out what the propagation path was. Other methods known as “rich feedback” have also been proposed. Further, U.S. patent application Ser. No. 12/478,564 discloses another method, where the channel observed at the receiver at one frequency is translated to the channel to be expected by the collocated transmitter at a different frequency. This method involves determining the number and parameters of a large number of scatterers in the environment by analyzing the received signal in the delay/Doppler domains. However, if the transmitted pilot symbols are distorted in phase and/or amplitude, the solution in the '564 application will give rise to phantom scatterers that are invalid for both the reception and transmission channel.
Even when the uplink and downlink signals use the same frequency, e.g., with Time Division Duplex (TDD) or ping-pong systems, reciprocity issues may further prevent the receiver from providing useful channel information to the transmitter for use on the same frequency. For example, a mobile device and a network station may be manufactured by different manufacturers. Thus, the uplink path includes the effects of a first manufacturer's transmission filter response, while the downlink path includes the effects of a second manufacturer's transmission filter response. Unless the combination of the first manufacturer's transmitter with the second manufacturer's receiver exhibits the exact same amplitude/phase versus frequency characteristics as the second manufacturer's transmitter combined with the first manufacturer's receiver, the channel will not be reciprocal.
Thus, there remains a need for an OFDM transmitter that addresses the group delay and/or amplitude errors introduced by one or more transmission filters across an allocated frequency spectrum.